Polyphase filters are known. Such filters operate by multiplying selected phases, or samples, of a filter impulse response with samples of the input signal(s). Prior-art multichannel polyphase filters have synchronized the selected phases of the filter impulse response with the positions of a commutator of the filter. In prior-art multichannel polyphase filters, a given position of the commutator has corresponded uniquely to a predetermined phase of the filter impulse response. Indeed, those of skill in the art of polyphase filter design have simply accepted that in multisubchannel polyphase modulators, the subchannel sampling rate is an integral multiple of the input sample rate, which implies that the channel bandwidth has to be an integer multiple of the input sample rate. Thus, in prior-art multisubchannel modulators and demodulators using polyphase filters, the subchannel bandwidth can either be the same as the input sample rate, producing subchannels too closely spaced for a receiver to adequately filter out adjacent subchannels, or the subchannel bandwidth can be twice the input sample rate, thus wasting spectrum with overly-separated subchannels.
For the above reasons, multichannel polyphase filters have not been considered a good choice for multisubchannel modulators and demodulators, where a subchannel bandwidth of slightly more, e.g., 12 percent more, than the input sample rate is desirable to prevent adjacent subchannel interference without wasting spectrum. On the other hand, the polyphase filter is known to be one of the most efficient ways for obtaining a sampling rate change.
Thus, what is needed is an apparatus for performing a non-integer sampling rate change in a multichannel polyphase filter. The apparatus preferably will enable an efficient multisubchannel polyphase modulator/demodulator that has superior flexibility of subchannel spacing.